June, 1919] Variations in Transpiration 507 



rhythmic rise and fall similar to those from other plants the 

 first day in the dark-room, but there are no indications of this 

 rhythm in the rate of water loss from the moth mullein. 



Experiment 8: To find out if the absence of a rhythm in the 

 transpiration curve of moth mullein was a characteristic of that 

 species or simply of an individual of the species, two more moth 

 mullein plants and a tobacco plant were placed in the dark- 

 room and their transpiration curves determined. But as in 

 Experiment 7 under constant environmental conditions both of 

 the moth mullein plants failed to indicate any rhythmic rise 

 and fall in the rate of water loss, which showed a gradual 

 decrease throughout the experiment. 



Experiment 9: In this experiment the transpiration curves 

 from two tobacco plants were obtained by placing Nicotiana 

 No. 1 in the dark-room after sundown and Nicotiana No. 2 about 

 noon of the preceding day. There was the usual rhythm in the 

 transpiration curve in Nicotiana No. 1, but Nicotiana No. 2 

 showed no indications of this rhythm. 



Summary of Results. 



The experiments given in this paper show that the differences 

 between the night and day rates of water loss from mullein and 

 tobacco are largely due to the differences in diffusion through 

 the stomatal pores. Transpiration from the leaves at night is 

 entirely cuticular and its rate is controlled by the temperature 

 and humidity of the air when the stomata are closed. But the 

 day rate is controlled by a number of factors operating to 

 increase or decrease it. When the stomata open the diffusion 

 gradient, or the difference between saturation deficit of the 

 intercellular spaces and the atmospheric saturation deficit, 

 causes a sudden rise in the rate of water loss. This rate con- 

 tinues until the leaf water deficit probably decreases the 

 diffusion gradient by increasing the resistance of the leaf to 

 water loss. After the leaf water deficit reaches a certain point 

 there are two factors, leaf water deficit and decreasing stomatal 

 pores, operating to reduce water loss. There is only one factor, 

 diffusion gradient, tending to increase it. This results in a 

 rounded curve as shown in figure 9. After the saturation 

 deficit reaches a maximum all three factors operate to decrease 

 the rate of water loss. This results in a rapid decline in the rate, 



